This invention relates to a method for repulping fibrous sheet materials such as paper or paperboard which contain wet strength agents. Specifically, the invention is directed to the repulping of paper and paperboard fibers which have been treated with polyamide wet strength resins.
Polyamides are widely used in the paper industry as wet strength agents for treating fibrous materials such as paper, paperboard, etc. However such polyamide resin-treated fibrous materials are very difficult to repulp because of the structural stability of the polyamide chain. Polyamides used as wet strength agents in paper and paperboard products are commonly derived from adipic acid and diethylenetriamine followed by cross-linking with epichlorohydrin. Examples of such polyamides include Kymene, Kymene 557 and Kymene AK manufactured by Hercules, Inc.; Cascamid C2, C25 and C1600 manufactured by Borden Chemical; and Disco Strength 5800 manufactured by Callaway Chemical.
Present methods for repulping fibrous materials containing polyamide wet strength resins require extreme conditions such as a pH of 10 or greater and temperatures of 160.degree. F. or more, or they require the presence of strong oxidizing agents.
Repulping fibers at high pH and high temperature is unsatisfactory for several reasons. Because papermaking machines are generally operated at a neutral or near neutral pH, if repulped fibers prepared at high pH are used to prepare recycled papers, a pH adjustment with acids would be required. High pH can cause damage to the repulped fiber and furthermore any adjustment of pH requires an additional step in the repulping method which is undesirable from an operational standpoint. Elevated temperatures are undesirable because they add increased energy costs to the method.
Other methods for repulping polyamide treated papers require the use of strong oxidizing agents to degrade the polyamides. Generally these other methods do not require the extreme operating conditions of high pH and high temperature, but such strong oxidizing agents may cause the formation and production of undesirable byproducts.
Melamine-formaldehyde polymers have also been employed as wet strength agents for fibrous materials. Fibers treated with melamine-formaldehyde resins do not generally show the difficulties associated with polyamides in the repulping of the fiber, however, questions associated with the safety of the use formaldehyde in making theses resins makes the use of melamine-formaldehyde resins undesirable. Efforts have been undertaken to design wet strength resins which are compatible with repulping operations, however, the resins which have been designed have not been completely satisfactory.
As a result of the difficulties encountered repulping papers containing polyamide wet strength agents, these papers are often not recycled. Therefore, a method is highly desirable wherein papers containing polyamide wet strength resins can be easily repulped.